The invention relates to a hydraulic screw pump and more particularly to a hydraulic screw pump capable of feeding waste water.
German Patent 34 44 440 discloses a hydraulic screw pump for feeding waste water. The screw is installed within a trough, with a gap provided between the water-carrying trough and the screw. The gap is very small and, to act as a seal, must be equal or even in all places along the length of the pump. The functioning of the screw depends to a great extent on meeting this condition. The width of the gap is the difference between the outside diameter of the screw and the inside diameter of the trough, on the condition that the axes of the screw and trough are aligned. Methods for manufacturing wear-resistant troughs with small tolerances in their diameters are disclosed in said German patent 34 44 440 and also in Abwassertechnik, 1987, Nr. 2, p. 52-53.
Such narrow tolerances for the outside diameter of a screw were not possible before, because these cylinders and screws are made from metal plate, and are welded joint products. Furthermore, over time, improper and unwanted extensions of the gap result from wear of the screw and the trough. The sealing or a packing material provided on the outer circumference of the screw, as disclosed in the prior art, such as U.S. Pat. No. 4,772,177, is not applicable. A gap is needed between trough and screw, the gap serving to avoid wear and friction.